With the development of mobile communication technology and tie progress of communication networks, various types of mobile communication systems have been developed. Further, International Mobile Telecommunications (IMT)-2000 systems have been developed to solve a global roaming problem between mobile communication systems caused by the development of various types of mobile communication systems. IMT-2000 systems are classified into a Code Division Multiple Access (CDMA) 2000-based synchronous mode system and a Wideband CDMA (WCDMA)-based asynchronous mode system.
Further, a mobile communication terminal (multi-mode multi-band terminal) capable of being used in both synchronous and asynchronous mode systems has been developed so as to support global roaming between mobile communication systems. By using such a mobile communication terminal, different types of services can be used in an asynchronous mode system area and a synchronous mode system area.
Currently, an asynchronous mobile communication system is being constructed in areas having a great number of service requests, so that a synchronous mode mobile communication system has been developed into a form in which a service area thereof includes service areas of an asynchronous mode system. During this process, when a user moves between an asynchronous mobile communication system and a synchronous mobile communication system, handover between the systems is required to provide continuous service.
For handover from the asynchronous mobile communication system to the synchronous mobile communication system, it is important to effectively detect a signal of the synchronous mobile communication system during the use of service provided by the asynchronous mobile communication system. For this operation, in order to perform handover, a multi-mode multi-band mobile communication terminal must drive a modem to interface with the synchronous mobile communication system while using the service provided by the asynchronous mobile communication system and must search for a cell in which to perform handover.
However, since the asynchronous and synchronous mobile communication systems use different communication methods, it is very difficult to implement handover between systems, and there is a limitation in increasing the probability of successful handover using only handover methods that have been proposed. Further, since a multi-mode multi-band mobile communication terminal is provided with two modems, it is greatly important to determine time points at which the two modems are to be driven or turned off.
In detail, a multi-mode multi-band mobile communication terminal requires handover during the use of service provided by an asynchronous mobile communication system, requires a time of about 10 seconds to drive a synchronous modem unit used to communicate with a synchronous mobile communication system, and must drive the synchronous modem unit before communication with the asynchronous mobile communication system is interrupted. There are disadvantages in that, if the synchronous modem unit is not driven before the multi-mode multi-band mobile communication terminal completely leaves an asynchronous mobile communication system area, handover is not successfully performed and a call is interrupted, while if the synchronous modem unit is driven unnecessarily early, two modems are simultaneously turned on, so that power consumption is increased. In this way, since the time point at which the synchronous modem unit is to be driven cannot be exactly known, problems, such as communication interruption occurring at the time of handover and increased power consumption, occur.
As described above, research into a method of controlling a multi-mode multi-band mobile communication terminal when performing handover is currently insufficient, so that there are disadvantages in that it is difficult to provide continuous service and the time required for the handover is greatly increased at the time of handover from an asynchronous mobile communication system to a synchronous mobile communication system.